Whisker-free lead frames

ABSTRACT

The electrical and mechanical properties of structures such as lead frames and other electrical/electronic devices containing, during processing, copper/tin interfaces are improved by introduction of nickel to such interface. Typically, a weight percentage of nickel to tin in the range 1 to 12 weight percent yields upon melting of the tin, an intermetallic compound with essentially no occluded, unbound tin. Thus undesirable anomalous structures such as tin needles and substantially non-planar interface compositions are avoided. Advantageously a nickel/tin/copper intermetallic interface that is substantially planar is formed in the substantial absence of needle-like tin structures.

TECHNICAL FIELD

This invention relates to tin containing interconnects for electronicand electrical devices and in particular to tin containing interconnectsthat avoid formation of tin whiskers.

BACKGROUND OF THE INVENTION

For electrical and electronic devices, typically electrical connectionsare made through the expedient of a lead frame or other coppercontaining structures. For example, in the fabrication of integratedcircuits, a silicon body having electronic circuitry is connected to ametal e.g. copper, lead frame such as shown in FIG. 1 at 2 with the chippositioned at 3 and connections between bonding pads on the chip and thelead frame shown at 4. After the chip is bonded to the lead frame, thechip is encapsulated typically in a polymer composition. The strip, 6,in FIG. 1 is removed from the lead frame in a process generallydenominated trimming. Thus, after trimming, the individual leads, 7, areno longer mechanically connected on one end. The leads are then bent tofacilitate connection to other electronic or electrical bodies such as acircuit board. Generally the bending involves the formation of at leastone curve such as shown in FIG. 2 for typical interconnection of anintegrated circuit with a circuit board.

For many applications, the interconnection between the lead frame andanother electronic or electrical entity is formed using a lead/tinsolder alloy. However, such alloy does not readily wet copper.Therefore, the copper leads are typically coated, e.g. plated, with alayer of tin to enhance wetting of the leads by solders before trimming.Although the tin layer does in fact facilitate wetting of the copperleads, other problems are generated. In particular, there is a tendencyto form long needle-like tin structures generally denominated whiskers.These structures are usually from 20 to 100 μm in length and can grow toas long as 1 mm or more. (The whiskers are most often single crystalstructures, but multi-crystal whiskers are also possible.) The exactinteraction between the copper and tin producing such crystallites isnot precisely known. It has been postulated that copper and tin form anintermetallic material in a manner that leads to regions of excess tin.These regions, it is contemplated, are under compressive stress,particularly at the curved sections of the lead frame after bending. Thecombination of excess tin and compressive stress enhances the tendencyto form whisker structures. The occurrence of, and thus the problemassociated with, whiskers are exacerbated because they also form when Snis plated on brass, alloy 42 and other commonly used electronicinterconnect metallization structures.

Decades ago, it was found that if, elemental lead (Pb), is added to thetin coating, whisker formation is essentially eliminated. Thus the issueof whiskers has not imposed reliability risks on electronic devices withPb-doped tin plated leads. However, impending legislation particularlyin European countries prohibits the use of lead for many applicationsincluding some involving electronic and electrical devices. Thus, therehas been a substantial impetus to remove lead from the tin coating. Suchremoval has the potential for renewing whisker formation as an issue tobe considered.

For similar reasons, use of lead-free solder is also being promoted.Such solders melt generally at temperatures above 217° C., and inapplication, for process control reasons, are typically used attemperatures above 240° C. Since such temperatures exceed the meltingpoint of tin (approximately 232° C.), concerns about tin whiskers havebeen mitigated since such whiskers are melted during the solderingprocess. Accordingly, it would appear that the difficulties associatedwith whiskers such as inadvertent shorting of lead frames or blocking ofoptical paths for electro-optic devices need not be a substantialconcern.

SUMMARY OF THE INVENTION

Surprisingly it has been found that tin whiskers are present even aftersoldering with materials having been subjected to soldering temperaturesabove 232° C. Thus the issues associated with such whiskers remain withlead-free solders. By practice of the invention, such whiskers areessentially totally avoided without the expedient of adding lead to thetin coating of the lead frame. Such results are achieved in oneembodiment by forming on the lead frame a nickel/tin composition havingfrom 1 to 12 weight percent nickel relative to tin. Such compositionresults in formation of an intermetallic compound with copper such thatessentially no free tin remains. Additionally, this copper/nickel/tinintermetallic has an essentially planar surface and thus is mechanicallyquite stable. As a result, tin whiskers are avoided and the resultantsolder connection is mechanically robust.

Additionally, the mechanical stability afforded by use of the inventionis useful even in applications where whisker formation is not aconsideration. For example, in certain applications, solder regions onan integrated circuit package (regions generally denominated bumpsand/or balls) are connected to pad regions on a circuit board. Suchconnection is accomplished through processes such as by aligning regionson the die to corresponding regions on the package substrate withsubsequent cohesion of the aligned regions. In one common lead freeapplication, a tin/silver, tin/copper, or tin/silver/copper solder alloyis used to connect an aluminum pad on an integrated circuit (IC) to themetal pad of a package substrate. To expedite this attachment thealuminum pad on the IC is coated with a solder friendly metal system.Such system includes an adhesion layer, a barrier layer and a solderwettable layer. (Examples of such solder friendly systems includetitanium/nickel-doped with vanadium/copper; aluminum/nickel-doped withvanadium/copper; chromium/chromium-copper/copper; electrolessnickel/immersion gold; and copper/nickel.) The metal substrate pad ispossibly bare copper but is often coated with materials such aselectroless nickel/immersion gold or electrolytic nickel/gold. Similarlythe substrate is connected to the board with a lead free solder alloy.Exemplary substrate metallization are copper, copper coated withelectroless nickel/immersion gold, and copper coated with electrolyticnickel/electrolytic gold.

The interaction between the tin of the solder and the copper from any ofthe solder pad metal interfaces (die, substrate, or board) has thepotential to produce mechanical instabilities with concomitantreliability issues often characterized by very non-planar interfaces.Through the use of nickel in the solder in the range of 1 to 12 weightpercent relative to tin, a robust planar intermetallic compound isformed that substantially reduces the proclivity to induce mechanicalinstabilities often arising when such an interface is not present. Thus,even in applications where whiskers are not a consideration, but wheretin materials promote mechanical instabilities the invention isnevertheless advantageously employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are illustrative of typical lead frame configurations;

FIGS. 3 through 5 exemplify structures involved in the invention; and

FIG. 6 demonstrates results achieved with the invention.

DETAILED DESCRIPTION

The presence of nickel in appropriate proportions to tin on a coppercontaining structure, such as those containing copper and/or copperalloy including brass, or lead frame materials such as alloy 42, avoidsproblems associated with whisker formation and substantially mitigatesmechanical weakness. To achieve such results, the nickel should bepresent with tin at an interface with copper so that the weight ofnickel relative to the tin is in the range 1 to 12 percent of the weightof tin present. Advantageously, the weight of nickel should be 3 to 7weight percent to that of tin and most advantageously 4 to 6 the weightpercentage of nickel to tin. Although an exact atomistic explanation forthe interaction of the nickel, tin, and copper is not presentlyresolved, it is contemplated that copper and nickel together with tinform an intermetallic compound and that compositions formed by employingtin and nickel in the appropriate proportions yield an essentiallycomplete reaction, i.e. consumption of the tin such that unreacted tinis essentially absent. Thus after reaction there is essentially noexcess elemental non-bound tin present at an interface with copper.Since elemental non-bound tin is required to form whiskers, whiskers areprecluded. Additionally, the complete reaction of the tin yields asubstantially planar surface for the copper/nickel/tin intermetalliccompound and thus a stronger mechanical bond.

For the desired intermetallic composition to form, the tin should bemelted, for example, during a soldering process. Since the melting pointof copper is 1,083° C. and the melting point of nickel is 1,453° C.,melting of these metals for essentially all electronic and electricalapplications is not practical. Thus, before melting, the three metalsshould be positioned geometrically so that their interaction to form anintermetallic compound is possible through diffusion at temperaturesnear the melting point of tin, i.e. temperatures in the range of about232° C. to 270° C. where significant degradation of organic substratesand/ or packaging materials is avoided. Since nickel is a slow diffuserin tin, it is typically disadvantageous to have a structure beforemelting such as shown in FIG. 3. As shown in FIG. 3, when the tin, 22,melts, nickel, 23, must diffuse through the molten tin to interact withthe copper, 24. (If the time is sufficiently long, e.g. in the range 60to 600 seconds and the temperature sufficiently high, e.g. 232° C. to300° C., then such a structure is useful but relatively inconvenient toemploy.) In contrast the structure shown in FIG. 4 is preferred since attemperatures around 240° C. copper, 34, and nickel, 32, interact withthe tin, 33 rapidly forming an intermetallic of the correct compositionwith a planar surface.

Generally, for the copper diffusion through and nickel consumption inmolten tin to be advantageous in a reasonable period of time, the nickelregion between the tin and copper should have a thickness in the range0.05 um to 1.0 um. Thicknesses less than 0.05 um often contain porositywhich promote uncontrolled reactions and non-planar boundaries.Furthermore, such nickel thicknesses require corresponding tinthicknesses that are prone to damage during the trim and lead bendingprocess. Thicknesses greater than approximately 1 um generally yieldinadequate nickel/copper interaction with the tin under typicalelectrical device thermal excursions. Alternatively, it is possible toplate or otherwise form an alloy of tin and nickel in the appropriateweight percentages onto the copper containing structure. (Thecombination of nickel and tin need not be an alloy. It is possible touse other nickel/tin compositions such as nickel/tin/silver,nicke/tin/silver/copper, or nickel/tin/copper.) The presence of othermaterials such as those typically used to modify the properties ofcopper are not precluded. For example, copper is often alloyed withmaterials such as iron to enhance properties such as mechanicalstability. The presence of such modifying materials generally does notpreclude the advantages of the invention.

The method of forming the desired nickel and tin regions is notcritical. Techniques such as vapor deposition, physical deposition,electroplating, or paste printing lead to useful results. As discussed,the invention relies on the appropriate presence of tin/nickel/copper atan interface where whiskers are to be avoided and/or mechanicalstability is required. Although a composition of nickel/tin having aproportion in the range 1 to 12 weight percent produces the desiredresult, it is possible to include in the final device structure otherregions of tin remote from a copper interface. For example, as shown inFIG. 5, an aluminum pad, 41, is overlaid by a region of nickel, 42, andcopper, 43. The copper is interfaced with a region, 44, having theappropriate weight percentage of nickel to tin. This region in turn isoverlaid by a larger region of tin-based solder, 45. Since the nickel ispresent at the copper interface upon melting, the appropriateproportions for copper/nickel/tin intermetallic having the desiredproperty is maintained by applying a thermal treatment generallyinvolving temperatures greater than 232° C. for times greater than 5seconds to insure that the proper composition of nickel and tin areprovided at the copper interface.

To produce the desired intermetallic and to avoid excess tin at a copperinterface, the structure should be heated to a temperature that allowstin to melt. Typically, temperatures in the range 232° C. to 270° C. areemployed. At temperatures below, 232° C. no substantial tin meltingoccurs, while at temperatures above 270° C, degradation of typicalpackaging polymers occurs. However, if employed on ceramic, metal, ormetal/ceramic packages, temperatures up to 500° C. may be used withoutunacceptable degradation of the package materials. Typically the desiredintermetallic material is formed in a time period between 5 and 120seconds for temperatures in the advantageous range. Thus, generally, thetemperature should be maintained in the desired range for such timeperiods. Times less than 5 seconds are undesirable since inadequatereaction to form the desired intermetallic is a frequent occurrence,while time periods greater than 120 seconds, although not precluded, aretypically not economic.

Although not required, it is possible to anneal the structure afterformation of the desired intermetallic composition. Generally, annealingtemperatures in the range 100° C. to 200° C. are useful in conjunctionwith annealing times in the range 0.5 hours to 8 hours. Subsequentprocessing of the device after intermetallic formation in the desiredannealing range eliminates the need for a specific annealing step.However, generally, processing at temperatures above 270° C. should beavoided with organic packaging materials due to their excessivedegradation.

As discussed, an intermetallic compound is formed at a copper containinginterface in an interconnection. This intermetallic material in oneembodiment is characterized by a percentage of unbound occluded tin lessthan approximately 15 weight percent relative to the total free Sncontent in the interconnection. (In one advantageous embodiment theoccluded tin is essentially absent.) The intermetallic composition isalso characterized in one embodiment by a surface having a planarity ofat least ÷0.9 um. (Planarity is this context is defined as maximumvariation in thickness from peak to valley across the intermetallicinterface.) The following example exemplifies processes, conditions, andcompositions involved in the subject invention.

EXAMPLE

A 0.25 um thick nickel layer was electroplated onto a copper lead frame.Subsequently a 3 um thick tin layer was electroplated onto the nickellayer. On a weight percentage basis the sample contains approximately 4%nickel and 96% tin. The device was then subjected to a 150° C. 1 houranneal. Finally the device was subjected to a typical solder reflowprocess with a peak temperature of 260° C. The time the device was at260° C. was approximately 18 seconds. A secondary electron image wastaken from a focused ion beam cross section of the sample. As shown inFIG. 6, the entire tin layer is converted into a planarnickel/copper/tin intermetallic layer 61 on the copper lead 62. Theplatinum layer 63 was deposited onto the sample prior to focused ionbeam cutting and acts as a reference that defines the surface of thetin/Ni/Cu layer.

1. A process of fabricating an interconnection between a first regioncomprising copper and a second conducting region wherein tin is presentat an interface with said first region during said process characterizedin that nickel is present with said tin wherein the weight percentage ofsaid nickel relative to said tin at said interface is in the range 1 to12 weight percent and the tin material is melted to induce formation ofa nickel/tin/copper intermetallic composition at said interface withsaid copper.
 2. The process of claim 1 wherein said melting is inducedby subjecting said connection to a temperature in the range 232° C. to270° C.
 3. The process of claim 2 wherein said subjecting is continuedfor a period of 5 to 120 seconds.
 4. The process of claim 1 wherein saidweight percentage is in the range 3 to 7 weight percent.
 5. The processof claim 4 wherein said weight percentage is in the range 4 to 6percent.
 6. The process of claim 1 wherein said interconnectioncomprises a lead frame.
 7. The process of claim 6 wherein said leadframe includes a flip chip bump.
 8. The process of claim 1 wherein saidinterconnection includes a region of copper as part of an electricalpad.
 9. The process of claim 1 wherein said interconnection comprises anelectrical interconnection.
 10. A structure comprising aninterconnection between a first region comprising copper and a secondelectrically conducting region characterized in that a region of anintermetallic composition comprising copper, nickel, and tin is presentbetween said first region and said second region and wherein the surfaceof said region of intermetallic composition includes less than about 15weight percent occluded tin relative to the total free tin content insaid interconnect.
 11. The structure of claim 10 wherein said structurecomprises an interconnection between a lead frame and a circuit board.12. The structure of claim 10 wherein said structure comprises aninterconnection between a flip chip and an electrical pad.
 13. Thestructure of claim 10 wherein said intermetallic compound is essentiallydevised of occluded tin.
 14. The structure of claim 10 wherein saidinterconnect comprises an electrical interconnect.